1. Field of the Invention
This invention pertains generally to circuit interrupters and, more particularly, to calibration of circuit breakers including a thermal trip assembly. The invention also relates to methods of thermally calibrating circuit interrupters.
2. Background Information
Electrical switching apparatus, such as circuit interrupters, include an operating mechanism and a trip mechanism, such as a thermal trip assembly and/or a magnetic trip assembly. For example, the trip mechanism is automatically releasable to effect tripping operations and manually resettable following tripping operations.
Examples of circuit breakers including trip mechanisms are disclosed in U.S. Pat. Nos. 5,805,038 and 6,838,961, which are incorporated by reference herein. Such circuit breakers, commonly referred to as “miniature circuit breakers,” have been in use for many years and their design has been refined to provide an effective, reliable circuit breaker which can be easily and economically manufactured and tested. As such, the ease of test of such circuit breakers is of importance.
As is well known, circuit breakers of this type include, for example, at least one set of separable contacts disposed within a non-conductive housing. Typically, there is a fixed contact attached to the housing and a movable contact coupled to the operating mechanism. The operating mechanism includes a movable handle that extends outside of the housing. Movement of the separable contacts is accomplished by the operating mechanism. The operating mechanism typically includes components such as the previously mentioned handle, an operating arm, upon which the movable contact is disposed, a cradle, and the trip mechanism, such as the previously mentioned thermal trip assembly and/or magnetic trip assembly. The cradle is coupled to a spring and disposed between the trip mechanism and the operating arm. The components may further include a frame to which the other components are coupled.
Referring to FIGS. 1 and 2, a circuit breaker 2 is magnetically tripped automatically, and instantaneously, in response to overload currents above a predetermined value higher than a first predetermined value for a thermal trip. Flow of overload current above a second, higher predetermined value through a bimetal 4 induces magnetic flux around such bimetal. This flux is concentrated by a magnetic yoke 6 toward an armature 8. An overload current above the higher predetermined value generates a magnetic force of such a strength that the armature 8 is attracted toward the magnetic yoke 6 resulting in the flexing of a spring 10 permitting the armature 8 to move to the right (with respect to FIGS. 1 and 2) to release a cradle 11 (partially shown in phantom line drawing) and trip the circuit breaker 2 open in the same manner as will be discussed below in connection with a thermal tripping operation.
Typically, a circuit interrupter, such as the circuit breaker 2, which includes a thermal trip assembly such as bimetal assembly 22, prior to thermal calibration has a relatively high thermal response (i.e., it takes relatively longer to trip). Still referring to FIGS. 1 and 2, during thermal calibration, a flat bit 12 from a circuit breaker calibration machine 14 (shown in block form) enters circuit breaker frame 16 through a slot 18 therein. The flat bit 12 is rotated clockwise (with respect to FIG. 2), thereby deforming the frame 16, as shown. The result is that the thermal trip time is reduced to a desired time (e.g., within a range of suitable time limits).
For example, the starting angle 20 of the bimetal assembly 22 is, for example and without limitation, 8.7° before calibration. As the flat bit 12 enters the calibration slot 18 in the frame 16 and begins to turn clockwise (with respect to FIG. 2), the upper right (with respect to FIG. 2) portion 24 of the frame 16 is deformed left and counterclockwise (with respect to FIG. 2). The geometry of the frame 16 is such that relatively thin sections 26,28 are designed into the frame 16, in order that the force, which is applied by the flat bit 12 to the right side 30 of the calibration slot 18 effectively decreases the starting angle 20 (FIG. 1) to the angle 20′ of FIG. 2, thereby rotating the bimetal assembly 22 counterclockwise (with respect to FIG. 2). This frame deformation decreases the starting angle 20 (FIG. 1) of the bimetal assembly 22 and lowers the thermal calibration of the circuit breaker 2.
In particular, the flat bit 12 deforms the upper right portion 24 of the frame 16 to the left (with respect to FIG. 2) and pivots the bimetal 4 (and the armature 8) in the opposite counterclockwise direction 31 (with respect to FIG. 2). This causes the circuit breaker 2 to trip at relatively lower bimetal temperatures (i.e., lowers the I2R thermal calibration of the circuit breaker 2). The construction of the bimetal 4 is such that the low expansion side is on the right side (with respect to FIG. 2). As the bimetal 4 heats up, it starts to deflect and pull the latching surface 32 of the armature 8 toward a tripping condition in the counterclockwise direction 31 (with respect to FIG. 2). Decreasing the starting angle 20 (FIG. 1) of the bimetal 4 during calibration effectively reduces the deflection (i.e., the amount of heat) of the bimetal 4 needed to pull the latching surface 32 of the armature 8 from under the latching surface (not shown) of the cradle 11.
When the circuit breaker 2 is closed, a persistent overload current of a predetermined value causes the bimetal 4 to become heated and deflect to the right (with respect to FIGS. 1 and 2), in order to effect a time delayed thermal tripping operation. The armature 8, which is supported on the bimetal 4 by the leaf spring 10, is carried to the right with the bimetal 4 to release the cradle 11 and trip the circuit breaker 2 in a well known manner.
There is room for improvement in methods of thermally calibrating circuit interrupters.
There is also room for improvement in circuit interrupter test systems.